A surface acoustic wave filter, that is, a surface acoustic wave device using the characteristics of a surface acoustic wave propagated on a substrate, has features of being more small-sized and lighter in weight, with a filter characteristic of better shape factor, than a dielectric filter or a laminated LC filter. Because of such merits, the surface acoustic wave filter is now widely for use in a cellular phone, etc.
In particular, when the surface acoustic wave filter is employed in a radio frequency (RF) section of the cellular phone or the like, the surface acoustic wave filter is required to have characteristics of low loss and good shape factor, in order to obtain improved receiving sensitivity, reduced consumption power, and sufficient noise-level suppression in the vicinity of the passband.
To meet such requirements, a variety of techniques have been proposed against the surface acoustic wave device to enhance the characteristic when used as surface acoustic wave filter. One of these techniques has been applied to an interdigital transducer (IDT) having a pair of comb electrodes and electrode fingers mutually interpolated between the comb electrodes, which are formed on a piezoelectric substrate of rotated Y-cut, X-propagating LiTaO3. In such an IDT, by narrowing a gap length between the tips of the plurality of electrode fingers constituting the comb electrodes and the bus bars connecting the plurality of electrode fingers is narrowed, it becomes possible to suppress a surface skimming bulk wave (SSBW) produced in the above-mentioned gap area. Using this method, a filter having good flatness in the passing characteristic is obtainable. (See the official gazette of the Japanese Unexamined Patent Publication No. 2002-31436Patent:patent document 1)
In another technique, a piezoelectric substrate having an anisotropic factor (γ) lower than −1 in the propagation direction is used. By determining a ratio L/S, a ratio of an electrode width L of the electrode fingers in the comb electrode constituting an IDT to a space S between neighboring electrode fingers, as well as a film thickness of the bus bar, the surface acoustic wave energy is efficiently confined. Thus an improved filter characteristic with a reduced loss can be obtained. (See the official gazette of the Japanese Unexamined Patent Publication No. 2002-100952:patent document 2)
Further, there is a technique that has been proposed by the inventors of the present invention, to obtain both wide bandwidth and low loss, by specifying a ratio between the electrode film thickness and the surface acoustic wave wavelength, and also a ratio between the electrode width and the electrode pitch as having particular relations (See the official gazette of the Japanese Unexamined Patent Publication No. 2002-176333:patent document 3).
Also, in the prior Japanese patent application No. 2001-390707 filed by the present applicant, a technique for obtaining a filter of good shape factor has been proposed. By specifying a particular relation in a ratio of the electrode width against the electrode pitch in a surface acoustic wave filter constituting a ladder filter, a filter of good shape factor is achieved.
Here, by sufficiently confining the wave energy in the transverse direction (i.e. perpendicular to the propagation direction) of the surface acoustic wave produced in an IDT, it becomes possible to obtain a filter having improved performance of loss and shape factor. The following aspects may be considered as a technique for such confinement of the wave energy in the transverse direction.
In one aspect, difference between the acoustic wave velocities in the bus bar and in the comb electrode is utilized, and the electrode thickness of the bus bar is increased, so as to obtain a reduced acoustic wave velocity, in case of using a leaky surface acoustic wave (LSAW) traveling on the substrate of rotated Y-cut, X-propagating LiTaO3 (refer to Patent document 2). Or, in another aspect, a dummy electrode is provided (Patent document 1).
When the energy propagating in the transverse direction is confined using the above-mentioned technique, in one sense, an improved filter loss is obtained. However, according to the survey and study carried out by the inventors of the present invention, the acoustic wave energy is concentrated at the tip gap of the bus bar and the comb electrode. Through our survey, it has been turned out that this concentrated energy results in an insertion loss generated in the filter.
Also, as another aspect, a technique has been disclosed in the aforementioned patent application: Japanese Patent Application No. 2001-390707. According to the technique, in a series arm resonator constituted of a ladder filter, a ratio L/S, i.e. ratio of an electrode width of a comb electrode to a distance between the electrodes, is set to approximately 0.35, to increase the acoustic wave velocity of the comb electrode. In addition, a dummy electrode is provided to confine the energy, thereby improving the passband characteristic particularly on the high frequency side. However, even in such a technique, the surface acoustic wave energy is concentrated at the tip gap. A similar problem occurs in a multimode filter, even when increasing the ratio L/S of the electrode width to the distance between the electrodes of the comb electrode, and at the same time, employing a dummy electrode.